Acoustic wave filters employ generally periodic arrays of electrodes configured to provide discrete elements such as transducers (for converting electrical to mechanical energy and vice versa), reflectors (for reversing the direction of propagation of an acoustic wave) and gratings (e.g., for separating transducers and/or resonant cavities and/or providing electrical isolation therebetween). These elements are grouped in a generally in-line configuration (e.g., reflector, transducer, grating, transducer, reflector) and are separated by inter-element gaps, with the entire array providing an electrical filtering function associated with the electrical port(s) of the transducer(s). The relative widths of the inter-element gaps affect: the electrical and acoustic performance of the composite filter.
The desired electrical performance often dictates gap sizes representing deviations from the periodicities of the respective elements, providing surface boundary condition discontinuities and reflecting a portion of the incident acoustic wave and scattering another portion into bulk acoustic waves. Particularly on high electromechanical coupling coefficient substrates, surface boundary condition discontinuities lead to bandwidth perturbations, increased in-band (i.e., passband) insertion loss and decreased out-of-band (i.e., stop-band) signal rejection.
What is needed is an apparatus and a method for making acoustic wave filters having electrical performance characteristics that are less degraded by bulk scattering losses even when implemented on high coupling coefficient substrates.